Could left-arm injury explain Haralson’s absence?

(Update: Haralson has been placed on season-ending injured reserve. Clark Haggans received a one-year deal, the 49ers announced.)

Linebacker Parys Haralson appeared to injure his left arm during the 49ers’ Aug. 26 exhibition at Denver, perhaps explaining why he hasn’t been seen in two practices and the exhibition finale since then.

Coach Jim Harbaugh would only allow that Haralson is “working through something” when asked about his status after Thursday night’s game against the San Diego Chargers.

The 49ers signed outside linebacker Clark Haggans on Sunday — the move hasn’t been officially announced — so Haggans might be doubling as an injury replacement for Haralson and as an extra linebacker for a thin corps that otherwise features Ahmad Brooks and Aldon Smith.

Upon replay review of the Aug. 26 exhibition at Denver, Haralson appeared to get hurt 10 1/2 minutes before halftime. Haralson lined up on the right edge and engaged in a block with Broncos tight end Joel Dreesen. Knowshon Moreno cut inside away from Haralson, who was standing toe-to-toe with Dreesen when he suddenly looked at his left elbow (or bicep, more so than his wrist) and headed for the 49ers’ sideline. Michael Wilhoite replaced Haralson on the next play.

Haralson has started 58 of the 49ers’ past 59 games, including both of last season’s playoff games. A seventh-year veterean, Haralson has started 68 of the 86 games he’s played as a 49er.

After a 14-sack rookie season, Smith was projected to replace Haralson in the starting lineup this season. Smith indeed started in the exhibition opener but sustained a bruised hip and did not play in any other exhibitions; Smith looks healthy and is poised to start Sunday’s season opener at Green Bay.